Oceans of the Solar System - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 223

enough, there may have been some form of plate tectonics, and there-
fore ocean basins too.
As time passed, the young Sun converted more hydrogen to helium
and shone more brightly. The oceans of Venus warmed, and yielded
more water vapour to the atmosphere. This greenhouse gas, acting
in concert with carbon dioxide and probably other gases such as
methane, warmed the surface even further—and this drove ever more
evaporation from the surface of Venus. The water molecules rose
high into its atmosphere, into regions where ultraviolet radiation
could split them into their component atoms, to be carried off by
the solar wind.
As the water supplies grew smaller, and the Venusian oceans shrank,
the opportunities to remove carbon dioxide from the atmosphere by
rainfall and then converting it into Venusian limestone or Venusian
coal reduced, so the level of that greenhouse gas rose too. At some
point, the vicious circle turned past the point of no return. The water
disappeared, and levels of carbon dioxide kept rising, released by vol-
canoes or by the thermal destruction of any carbon-bearing rocks
that had managed to form. Today, that process has reached comple-
tion. The amounts of carbon in the atmosphere of Venus now are
roughly equivalent to that of the carbon stored underground on Earth
as limestone and coal.
Venus today lacks a strong, protective magnetic field and there-
fore easily loses any remaining water to the solar wind. Therefore,
was the breakdown of the magnetosphere a key step in the loss of
Venusian oceans? It is likely that Venus once had a magnetosphere
like Earth's, as it accreted from similar materials to Earth and prob-
ably possesses an iron core. For part of its history, this iron core likely
generated a strong magnetic field, and for the first billion years or
so of Venus's history the atmosphere could have been protected
from the ionizing radiation of the solar wind. At some point the